1. Field of the Invention
The present invention relates to a method of fabricating a printed circuit board (PCB). More particularly, the present invention pertains to a method of fabricating a PCB, in which a substrate, where a copper foil is formed on a release film and a prepreg, is employed as a base substrate and a core insulating layer is removed after the fabrication of the PCB, thereby reducing the thickness of the final product.
2. Description of the Prior Art
In accordance with the trend of miniaturization and slimming of electronic goods, a size of a package tends to be reduced. In this regard, the total size of the package depends on the size of a substrate used in the package.
FIGS. 1a to 1m are sectional views illustrating the stepwise fabrication of a six-layered PCB in the conventional build-up manner. In the specification of the present invention, the term “build-up manner” means a process which comprises forming internal layers and layering external layers one by one on the internal layers.
FIG. 1a is a sectional view of an unprocessed copper clad laminate (CCL) 101. Copper foils 102 are applied onto an insulating layer 103. Generally, the copper clad laminate acts as a substrate of a PCB, and means a thin laminate consisting of the insulating layer onto which copper is thinly applied.
The copper clad laminate is classified into a glass/epoxy CCL, a heat-resistant resin CCL, a paper/phenol CCL, a high-frequency CCL, a flexible CCL (polyimide film), a complex CCL, and the like, in accordance with its use. Of them, the glass/epoxy CCL is most frequently used to fabricate double-sided PCBs and multilayer PCBs.
The glass/epoxy CCL consists of a reinforcing base substance in which an epoxy resin (combination of a resin and a curing agent) is penetrated into a glass fiber, and a copper foil. The glass/epoxy CCL is graded FR-1 to FR-5, as prescribed by the National Electrical Manufacturers Association (NEMA), in accordance with the kind of reinforcing base substance and heat resistance. Traditionally, the FR-4 grade of glass/epoxy CCL is most frequently used, but recently, the demand for the FR-5 grade of glass/epoxy CCL, which has improved glass transition temperature (Tg), is growing.
In FIG. 1b, the copper clad laminate 101 is drilled to form a via hole 104 for interlayer connection.
In FIG. 1c, electroless copper plating and electrolytic copper plating processes are conducted. In this regard, the electroless copper plating process is conducted before the electrolytic copper plating process. The reason that the electroless copper plating process is conducted before the electrolytic copper plating process is that the electrolytic copper plating process using electricity is not possible on the insulating layer. In other words, the electroless copper plating process is conducted as a pretreatment process to form a thin conductive film needed to conduct the electrolytic copper plating process. Since it is difficult to conduct the electroless copper plating process and to assure economic efficiency, it is preferable that a conductive part of a circuit pattern be formed using the electrolytic copper plating process.
Subsequently, a paste 106 is packed in the via hole 104 so as to protect electroless and electrolytic copper plating layers 105 formed on a wall of the via hole 104. The paste is generally made of an insulating ink material, but may be made of a conductive paste according to the intended use of the PCB. The conductive paste includes a mixture of any one metal, which is selected from Cu, Ag, Au, Sn, Pb, or an alloy thereof and acts as a main component, and an organic adhesive. However, the process of plugging the via hole 104 using the paste may be omitted according to the purpose of the MLB.
In FIG. 1c, for convenience of understanding, the electroless and electrolytic copper plating layers 105 are illustrated as one layer without distinguishing two layers from each other.
In FIG. 1d, an etching resist pattern 107 is constructed to form a circuit pattern for an internal circuit.
A circuit pattern, which is printed on an artwork film, must be transcribed on the substrate so as to form the resist pattern. The transcription may be conducted through various methods, but the most frequently used method is to transcribe a circuit pattern, which is printed on an artwork film, onto a photosensitive dry film using ultraviolet rays. Recently, a liquid photo resist (LPR) has sometimes been used instead of the dry film.
The dry film or LPR to which the circuit pattern is transferred acts as the etching resist 107, and when the substrate is dipped in an etching liquid as shown in FIG. 1e, the circuit pattern is formed.
After the formation of the circuit pattern, the appearance of the circuit pattern is observed using an automatic optical inspection (AOI) device so as to evaluate whether an internal circuit is correctly formed or not, and the resulting substrate is subjected to a surface treatment, such as a black oxide treatment.
The AOI device is used to automatically inspect the appearance of a PCB. The device automatically inspects the appearance of the PCB employing an image sensor and a pattern recognition technology using a computer. After reading information regarding the pattern of an objective circuit using the image sensor, the AOI device compares the information to reference data to evaluate whether defects have occurred or not.
The minimum value of an annular ring of a land (a portion of the PCB on which parts are to be mounted) and a ground state of a power source can be inspected by use of the AOI device. Furthermore, the width of the circuit pattern can be measured and the omission of a hole can be detected. However, it is impossible to inspect the internal state of a hole.
The black oxide treatment is conducted so as to improve adhesion strength and heat resistance before an internal layer having the circuit pattern is attached to an external layer.
In FIG. 1f, resin-coated copper (RCC) is applied to both sides of the resulting substrate. The RCC consists of a substrate in which a copper foil 109 is formed on only one side of a resin layer 108, and the resin layer 108 acts as an insulator between the circuit layers.
In FIG. 1g, a blind via hole 110 is formed to electrically connect the internal and external layers to each other. The blind via hole may be mechanically drilled. However, it is necessary to more precisely conduct the drilling in comparison with processing of a through hole, and thus, it is preferable to use an yttrium aluminum garnet (YAG) laser or a CO2 laser. The YAG laser can drill both a copper foil and an insulating layer, but the CO2 laser can drill only the insulating layer.
In FIG. 1h, an external layer 111 is laminated according to a plating process.
In FIG. 1i, the external layer 111 formed as shown in FIG. 1h is patterned according to the same procedure as the formation of the circuit pattern of the internal layer. The patterned external layer 111 is then inspected in terms of the circuit and subjected to a surface treatment, as in the case of the circuit pattern of the internal layer.
In FIG. 1j, additional RCC is applied to both sides of the resulting substrate. This RCC includes a resin layer 112 and a copper foil 113 coated on one side of the resin layer 112, and the resin layer 112 acts as an insulator.
In FIG. 1k, a blind via hole 114 is formed to electrically connect the external layers to each other using the laser as described above.
In FIG. 1l, the additional external layer 115 is laminated according to a plating process.
In FIG. 1m, the additional external layer 115 is patterned according to the same procedure as the external layer 111, and the circuits of the patterned external layer 115 are then inspected and the layer is subjected to a surface treatment.
The number of layers constituting the multilayer PCB may be continuously increased by repeating the lamination of layers, the construction of the circuit patterns, the inspection of the circuit patterns, and the surface treatment of the resulting structure.
Subsequently, a photo-solder resist and an Ni/Au layer are formed on the resulting circuit pattern, thereby creating a six-layered PCB.
In a conventional method of fabricating a substrate (PCB), an organic resin is used as an insulating layer to form a copper clad laminate (CCL) which is plated with Cu on both sides thereof. In this regard, the organic resin also acts as a backbone to support a product so as to keep its shape. Hence, it is necessary that a thickness of the product be 50 μm or more so as to keep the shape. In other words, when the thickness is the above limit or less, it is impossible to fabricate the product using a typical organic resin.
With respect to this, U.S. Pat. No. 6,696,764 discloses a method of fabricating two PCBs by cutting a central part of the PCB through a mechanical process after construction of the PCB. However, since the cutting is mechanically conducted, it has limited application to the fabrication of a precise PCB. Additionally, a core insulating layer remains after the cutting, making the PCB thick.
Accordingly, a more fundamental alternative proposal is needed to reduce a thickness of the PCB.